are similar in form to the monopolar membrane potential in counterion-only liquids [1]}, and the screening parameter \(\kappa _0\)  satisfies the transcendental equation
\(\frac{\kappa _0 a}{\sqrt{2}}\tan \left(\frac{\kappa _0 a}{\sqrt{2}}\right)=\frac{Q_+a}{2\mu _-}.\) 
